Read on for a journey through New York's geological time and how it has shaped the features it is most famous for today: the Adirondacks, the Catskills, Niagara Falls, the Finger Lakes, and the bedrock of New York City.
Part 1: The Geology of New York State Through Time
Basic Geologic Concepts
The crust is the uppermost and coldest layer of the Earth. Beneath it is the mantle and core. Because the crust is not particularly hot and because it experiences comparatively little pressure, the crust is relatively rigid and has broken up into a jigsaw puzzle of pieces known as tectonic plates. These tectonic plates “float” on the mantle beneath and move with the mantle’s convection currents which slowly push tectonic plates together or apart.
Two things can happen when tectonic plates are pushed together: 1) they collide, squeeze together, and push upwards to form a mountain (this is called an “orogenic event” or “orogeny” and has formed mountains like the Alps, Himalayas, and Appalachians) or 2) one slips underneath the other and melts deep underground to form magma which rises and creates a ring of volcanoes (e.g. the Cascades and the Andes). Both result in the formation of igneous and metamorphic rocks. Igneous rocks form from cooling magma which has found its way to the surface. Metamorphic rocks occur because intense heat or pressure has caused a rock’s chemical structure to change but has not caused the rock to melt. Additionally, both types of collisional events result in rock deformation such as rock folding and faulting (cracking).
New York has experienced multiple orogenic (aka. mountain-building) events throughout its geologic history. Mountains formed as tectonic plates collided and resulted in rock metamorphism, deformation, and orogeny-related volcanics (e.g. igneous intrusions). The 5 most recent orogenies are responsible for the formation of the Appalachians and the most recent orogeny occurred during the formation of Pangaea. Orogenies prior to these 5 have long since eroded away, although their metamorphosed rocks still make up the deep bedrock of New York, such as the Manhattan Prong, which lays below most of New York City’s boroughs. New York’s landscape today is, overall, the result of the formation of the Appalachians followed by millions of years of erosion and sediment deposition and, most recently, glacial activity (which led to further erosion and sediment deposition).
2.5 - 1.3 billion years ago: The formation of North America
The North American continent started as a collection of microcontinents - mini volcanic islands similar to Hawaii - which formed from hotspots. These collided as a result of tectonic plate motion and accreted (joined) with one another to form the North American craton, also known as Laurentia. The “craton” makes up the interior portion of the current American continent and is the oldest and most stable part of this continent. It took hundreds of millions of years for America to accrete enough material to become the continent we know today.
As the craton was assembling from colliding microcontinents, New York’s geographic location was underwater gathering layers of sand, mud, and clay as well as calcium carbonate minerals1. These eventually compressed from the weight of water and the continued addition of sediments to form sedimentary rock such as shale (from mud and clay), limestone (from calcium carbonate minerals), and sandstone (from sand).
1.3 billion years - 650 million years ago: The Grenville Orogeny (supercontinent Rodinia)
The Adirondacks, Thousand Islands, Hudson Highlands, & Central Park
Between 1.35 and 1 billion years ago, Laurentia was involved in the formation of Rodinia: an ancient supercontinent. As part of the tectonic plate cycle, continents periodically collide and then separate. Laurentia is no exception. Laurentia collided with another continental plate in what is now called the Grenville Orogeny. The collision created a large mountain range along Laurentia’s eastern coast, similar to the Alps today. In the process, it resulted in orogeny-related volcanics and metamorphosed the pre-existing sedimentary rock into metamorphic rocks such as slate and schist (from shale), marble (from limestone), quartzite (from sandstone), and gneiss (from schist or igneous rocks; gneiss forms when a rock experiences enough heat to partially melt). Although the mountain range has long since eroded away, Grenville rocks still make up the deep bedrock of New York. These billion-year-old rocks are visible on the surface in some areas such as the Adirondacks, Thousand Islands, and regions of New York City (such as the Hudson Highlands and Central Park) due to erosion.
Around 750 million years ago, Rodinia split apart and Laurentia became its own continent once again. By this point, the Grenville mountain range had almost completely eroded away.
575 - 455 million year ago: The Avalonian-Cadomian Orogeny
This period of time is geologically complicated and not completely understood. It involved more collisional and volcanic events that resulted from the break-apart of Rodinia and numerous other complicated interactions between the continents of the time. These events led to further metamorphism along New York’s coast and the addition of new igneous rock into the landscape.
More importantly, this period of time saw New York almost fully submerged under the ancient Potsdam Sea. Under this sea, much of the Cambrian Period (541-484 million years ago) and Ordovician Period (484-444 million years ago) sedimentary rock formed, some of which would be metamorphosed by later orogenic events. These metamorphosed sedimentary rocks are a large part of the Manhattan Prong region (underlying most of New York City).
455 - 66 million years ago: The Appalachians
Plus the Catskills, the split of Pangaea, and the Palisades
The formation of the Appalachians resulted from numerous orogenic events, all of which occurred after the Avalonian-Cadomian. The northern Appalachians formed from 5 distinct orogenies: the Taconic (460-450 million years ago), the Salinic (450-423), the Acadian (420-400), the Neoacadian (395-350 million years ago), and the Alleghanian (300-290 million years ago). These orogenies metamorphosed the sedimentary rock which formed during the Cambrian and Ordovician Periods; these are a large part of the Manhattan Prong underneath New York City. The Acadian orogeny resulted in the formation of the Catskills: a mountain range composed of mostly metamorphosed sedimentary rock. The most recent orogenic event - the Alleghanian - was the result of the formation of Earth’s most recent (and most well-known) supercontinent: Pangaea. Pangaea broke apart approximately 200 million years ago. During Pangaea’s break up, the splitting tectonic plates caused magma to more easily reach the surface of the earth. Near New York City, this allowed for a large horizontal layer of magma to intrude between layers of older rock and cool to form a thick layer of rock (geological name: a “sill”) known as “the Palisades.” This sill is most visible along New Jersey’s side of the Hudson river as a near-vertical cliff exposed due to the erosion of surrounding rock.
Overview: The Effect of Orogeny
Today, the North American continent stands alone. Its east coast is in a period of geologic calm and will not experience another orogenic event until the continents once again collide to form a future supercontinent. Its landscape, however, bears the scars (and mountains) of previous collisions.
Each orogenic event had a similar effect on New York’s geology. First: orogeny, and its resulting metamorphism, deformation, and igneous (volcanic) activity. Part of the effect of orogeny, which changed the weight and shape of the landscape, was the formation of faults: large cracks/fractures in the Earth’s crust. These cover New York’s surface. Second: the accretion (addition) of new, hard rock to the North American craton, expanding the North American continent Eastward. Last: the erosion of rock and the deposition of sediment which occurred in the geologic calm which followed an orogenic event.
2,600,000 - 10,000 years ago: The Last Ice Age
Finger Lakes, Niagara Falls, etc.
New York’s landscape has been most recently affected by the last Ice Age, a period of time during which glaciers periodically grew and advanced south and then melted and retreated north. The last glacial advance peaked ~20,000 years ago and, during this peak, ice covered almost all of New York. Periodic glacial advances and retreats softened New York’s topography, eroding higher regions and depositing sediments in lower regions. New York’s famous Finger Lakes are the result of glaciers gouging out and damming a series of streams, turning them into long lakes. The Thousand Island region was sculpted into its “knob (island) and hollow (water)” landscape by glaciers. The melting of the glaciers is responsible for releasing the large amounts of water which carved out the Niagara Escarpment and created Niagara Falls. Long Island is the result of glacial moraines (sediment which piles at the edge of advancing glaciers as they push southward) and outwash (sediment deposited by melting glacial water as they retreat) which accumulated within the ocean to the south of New York State.
Part 2: The Geology of the New York City Region
There are a surprising number of areas in which the bedrock of New York City can be seen at its surface. Central Park is, perhaps, one of the best examples. This 843 acre green space is located over a region of metamorphic rock known as the Manhattan Prong which formed during the Grenville and Appalachian-forming orogenies. The Manhattan Prong underlies the entirety of the Bronx and Manhattan, the upper part of Staten Island, and the western edge of Brooklyn and Queens. Figure 1(left) illustrates the general types and locations of Manhattan Prong rocks, including those that formed during the Appalachian-forming orogenies - Hartland Schist (green), Inwood Marble (yellow), and Manhattan Schist (red) - and those that formed during the Grenville Orogeny: Fordham Gneiss (blue). Out of the 4 rock types, all except the Inwood Marble are visible on the surface in Central Park. You can see them yourself using this field guide made by the American Museum of Natural History. The metamorphic bedrock of Manhattan and the Bronx is overlain by a relatively thin layer of glacial sediment (see Figure 2).
The only bedrock in Brooklyn and Queens consists of a sliver of the Manhattan Prong. Above this is loose sediment from the Cretaceous Period (145-66 million years ago) and glacial deposits. The Cretaceous sediments consist of sand and clay and are known as the Raritan Formation (the same as that of Staten Island in Figure 1 (right)). The glacial deposit sediments are mostly gravel. Long Island, where the two boroughs are located, formed due to glacial sediment deposition from glacial moraines and outwash. Glacial moraines are large amounts of sediment which build up on the edges of glaciers as they push across a landscape. When the glacier begins to melt, these sediments are left behind. Glacial outwash occurs as glaciers melt and release large amounts of water. This water carries sediments across long distances and, as it slows, deposits them.
Staten Island’s bedrock consists of serpentinite which intruded into the Manhattan Schist of the region, shale and sandstone sedimentary rocks from the Triassic Period (252-201 million years ago), and the Palisades igneous intrusion. Above this bedrock is the Cretaceous Raritan formation, consisting of sand and clay sediments, and glacial sediment deposits. The Arthur Kill region consists of human-made landfills.
Most of New York City’s bedrock has been covered by sediments, artificial fill, and other human-made features, making it difficult to see the extent of bedrock. Despite this, geologists have been able to make accurate maps of the region’s geology using exposed rock outcrops and more complicated methods such as coring. Exposed outcrops are mostly found on the surface in places such as parks. Due to tunnels and deep wells, exposed rocks can also be found underground. Where underground rocks aren’t already exposed due to human-made features, geologists can look at “cores” - long, cylindrical columns of rock or sediment which have been extracted from the earth by a drilling mechanism in order to analyze subsurface, often very deeply located, rocks. Together, the two methods allow for a relatively complete picture of New York’s geology. Next time you find yourself out for a walk in this city, think about all the geologic layers under your feet and the hundreds of millions of years which formed them.